Genetically Male Tilapia (GMT)

The Fish Genetics and Biotechnology Program (FGBP) of the Freshwater Aquaculture Center of Central Luzon State University (CLSU) is a research and development program for the genetic improvement and conservation of cultured freshwater fishes. Its activities include genetic manipulations, selective breeding and genetic characterization of cultured species and strains. Priority activities also include the packaging and extension of information on new technologies and the dissemination of improved fish.

One of the major undertakings of the FGBP is Phil-Fishgen, a project designed to disseminate the products of collaborative research on sex control in tilapia and to generate income for the financial sustainability of future FGBP research activities.

The Tilapia

Tilapia is a tropical fish species originating from Africa. Due to its popularity for aquaculture it has been introduced around the World and is widely cultured throughout the tropics and sub-tropics. Tilapia is a common name for a group of three genera, Oreochromis, Sarotherodon and Tilapia, with the Nile tilapia, Oreochromis niloticus, generally considered the best species for freshwater aquaculture. The fish has many attributes suited to domestication and culture including good flesh quality and flavor, wide tolerance of different environments, resistance to common fish diseases, and relative ease of reproduction in captivity.

This ease of reproduction actually represents one of the ; principal problems in the optimization of yields in tilapia culture, the fish breeds too readily. Energy is diverted from growth, into the behavioral and physiological interactions between the sexes, and into the production of eggs. The most effective solution to this problem is to grow only one sex, preferably males as they grow faster and to a larger size. A number of technologies have addressed this problem including hybridization and hormonal sex reversal, but none produce monosex fish in a consistently effective, affordable and environmentally safe way.

The YY Male Technology

Through the application of simple genetic manipulations, the UWS, in collaboration with FAC-CLSU, have developed an innovative and robust new genetic technology for producing all-or nearly all-male progeny in the Nile tilapia. Known as the “YY male technology” this takes the form of a breeding program combining feminization and progeny testing, to produce novel males with YY genotypes (i.e. with 2 male sex chromosomes) instead of the usual XY male genotype. These YY males are known as “supermales” and have the unique property of siring only male progeny.

Genetically Male Tilapia (GMT)

The all-male progeny of YY males are termed genetically male tilapia (GMT) and are normal XY males (although some can “naturally” revert to female, giving GMT an average sex ratio of >95% male). The hormone treatments used in the breeding program to produce YY males are two generations removed from the fish that are consumed so neither the GMT or their YY male parents are treated in any way. This is an environmentally friendly technology requiring no special facilities for application. A series oif on-farm trials of GMT were conducted in the Philippines, including all major types of culture system ranging from extensively managed earthen ponds through to intensive, tank based farms.

The GMT proved to have excellent properties for aquaculture, cost effectively producing significant increases in yield of uniform sized fish and controlling reproduction in all culture systems (see figure). YY males, which are as viable and fertile as normal males, can now be produced in large numbers and are being used commercially to mass produce high yielding GMT for tilapia growers in the Philippines, through a network of licensed and accredited hatcheries.

GMT produced higher yields, through a combination of enhanced survival and faster growth rates, in 11 the farms on which it was tested. These higher yields were accompanied by lower food conversion ratios and greater size uniformity, factors which also contributed to the improved profitability of culturing GMT compared to present available stocks. GMT have shown similar increases in performance in intensive culture systems in other countries.

For further information please contact:

PHIL-FISHGEN Freshwater Aquaculture Center
Central Luzon State University, Nueva Ecija
Telefax: (044) 456-0683
Email: [email protected]

Phil-Fishgen is an organization which is affiliated with the Freshwater Aquaculture Center (FAC) of Central Luzon State University (CLSU), the University of Wales Swansea and Fishgen Ltd. (U.K.) and it is established to disseminate the outputs of research on tilapia genetics in the Philippines. photo from

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